This invention relates to a method and apparatus for forming visible images, and in particular for forming visible images by the selective application of electrical potentials to an imaging material. A material in which visible images may be formed by chemically-produced color changes as a result of the selective use of electrical potentials, is hereinafter called an electrochromic material. With many electrochromic materials, the images are reversible, that is, they can be erased by applying a reverse potential to the imaging material.
Various imaging systems are known for reversibly forming visible images in this way. In one such system, a liquid electrochromic material is used as an imaging medium, and a set of first electrodes cooperate with one or more counter-electrodes to cause deposition of colored material onto selected ones of the first electrodes which have electrical potentials applied to them, to form an image. Typically, the first electrodes are transparent conductive members. Erasure is achieved either by the removal of the potentials and the passage of time, or by reversal of the potentials. One kind of electrode configuration which has been suggested for the first electrodes of such systems is the so-called 7-bar configuration, in which seven straight bars are arranged to form the general shape of a square-sided FIG. 8. By energizing selected ones of the bars, each of the numerals from 0 to 9 may be depicted.
In another known kind of electrode configuration, the electrodes used to selectively apply potentials across the material are in the form of two spaced sets of parallel conductive strips which overlie one another in an orthogonal matrix configuration. By applying potentials to the appropriate strip of each set, it is possible to address any point in the material defined by an intersection of two strips. By making each applied potential less than the coloration threshold potential for the material but greater than one half of the threshold potential, it is possible to cause coloration only at the selected intersection.
This latter configuration suffers from the disadvantage that in order to pass sufficient charge through the electrochromic material to cause coloration in a usefully short time, it is necessary to pass a large current pulse. Although this can be achieved, the necessary circuitry is expensive, and difficulty may be experienced in fabricating electrodes of sufficiently high conductivity to cope with such currents.
It is an object of the present invention to provide an improved imaging system which utilizes an electrochromic material and in which the above disadvantage is overcome. According to the present invention, there is provided a method of causing coloration of an electrochromic material comprising passing through the material an electrical pulse of a predetermined polarity which tends to cause coloration and at a potential above a coloration threshold potential for the material, and simultaneously or subsequently applying to said material a D.C. potential of said predetermined polarity and of magnitude less than said threshold potential.
According to another aspect of the invention, there is provided an apparatus for causing coloration of an electrochromic material comprising spaced electrodes in contact with an electrochromic material, means for passing between said electrodes an electrical pulse of a predetermined polarity which tends to cause coloration and at a potential above a coloration threshold potential for the material, and means for simultaneously or subsequently applying to said material a D.C. potential of said predetermined polarity and of magnitude less than said threshold potential.